Why is manga dying in popularity?

My friend who's a girl, loves manga. And one time she told me she felt bad that manga is doing better in the states than comics are. I laughed and told her why she thought that. She replied saying other than me, all of her friends love manga. I told her that's because she never bothered befriending people who like comics. She laughed and agreed. Well one time me and her were both curious and wanted to see which out performed the other. She was shocked to see that manga made only 0.90% (less than a percent) of annual sales in the US, while comics made up the rest with Marvel taking in 40%, DC taking in 30% and so on and so forth. We went to our local Barnes n' Noble book store, and asked the the workers there what Graphic Novel sold the most. Well all the workers at the desk were female and replied Vampire Knight and One Piece were the top sellers selling a total of 25 copies. I asked her if she had a copy of "Old Man Logan, and she checked. It turned out they ran out, and have been selling on average 5 copies a month from January till now which put it at 25. So I replied saying "that's funny so that one graphic novel from Marvel comics has sold what those two did combined." She said yeah. Then I asked about Green Lantern Secret Origin, a Graphic Novel from DC comics, and it sold 20. So just those two alone out performed their top Manga. They were surprised but admitted that they just noticed the manga because the covers were more colorful. So now my friend is asking, why is Manga dying not just in the US but world wide while comics have remained the same trend for the past 25 years (up and down sales)?